Which of the following cycloalkanes has the most angle strain?

• A. ${\mathrm{cyclopropane}}_{}$
• B. ${\mathrm{cyclopentane}}_{}$
• C. ${\mathrm{cyclobutane}}_{}$
• D. ${\mathrm{cyclohexane}}_{}$

Answer: (A)

Verified step by step guidance

1. Recall that angle strain in cycloalkanes arises when the bond angles deviate from the ideal tetrahedral angle of 109.5° due to ring size constraints.
2. Identify the ideal bond angles for each cycloalkane based on their ring size: cyclopropane (3-membered), cyclobutane (4-membered), cyclopentane (5-membered), and cyclohexane (6-membered).
3. Understand that smaller rings have more angle strain because their internal bond angles are forced to be much smaller than 109.5°. For example, cyclopropane has bond angles of approximately 60°, which is far from ideal.
4. Compare the bond angles: cyclopropane (~60°), cyclobutane (~90°), cyclopentane (~108°), and cyclohexane (~109.5°). The closer the bond angle is to 109.5°, the less angle strain the molecule has.
5. Conclude that cyclopropane has the most angle strain because its bond angles deviate the most from the ideal tetrahedral angle, making it the most strained among the given options.